To provide the best summary of the railroad system from the options given, it would help to analyze each statement against the context of railway development in the United States up until 1860.
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"The majority of the railroads that connected the North to the South were built by 1850."
- This statement may not be accurate as significant railroad construction connecting different regions occurred after 1850.
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"The railroads established by 1860 each began and ended at a major American city or port."
- While many railroads connected significant cities, it is not necessarily true for all railroads, especially smaller or less significant ones.
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"The primary direction of the railroads built between 1850 and 1860 was north to south."
- Generally, railroads predominantly connected east to west or followed geographic features rather than a strict north-south orientation.
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"The extent of the railroads in the South was very limited compared to that of the North."
- This statement is generally accurate. By 1860, the railway network in the North was much more developed and extensive compared to that of the South.
Based on this analysis, the best summary statement is:
"The extent of the railroads in the South was very limited compared to that of the North."